Am I, or my mother, obligated to pay back the price of the wool I took from my sister without her knowledge, and at what value (at the time of purchase or now)?

It is not permissible for a mother to take the money of one of her children without his permission to give it to another. It is obligatory for you to return the wool if it is still in existence; otherwise, you must return an equivalent amount, and if that is not possible, then its value. The value should be returned based on its highest value from the day it was taken until the time it perished. If the mother compensated the sister or appeased her, then nothing is required of you.